Only one small part of the enzyme, called the active site, accommodates the substrate (s).

Def: Region on the surface of an enzyme where the substrate binds and where the reaction occurs. 77.

active transport -80

During active transport, molecules or ions move through the plasma membrane, accumulating on one side of the cell.

Def: Use of a plasma membrane carrier protein to move a molecule or ion from a region of lower concentration to one of higher concentration; it opposes equilibrium and requires energy. 80

aquaporin - 79

Water, a polar molecule, moves across the membrane through channels called aquaporins.

Def: Membrane channel through which water moves by simple diffusion. 79

calorie -72

A calorie is the amount of heat required to raise the temperature of 1 gram of water by one degree Celsius. this isn't much energy, so the calorie value of food is listed in nutrition labels and in diet charts in terms of kilocalories (1000 calories)

Many metabloic reactions require energy. Energy can be supplied whn a reaction that requires energy ( like building a protein) occurs in the vicinity of a reaction that gives up energy (like ATP breakdown) A coupled reaction brings them together in such a way that the energy-releasing reaction can drive the energy-requiring reaction. Usually the energy-releasing reaction is ATP breakdown, which generally releases more energy than the amount consumed by the energy-requiring reaction. This increases entropy, but both reactions will proceed.

Def: Reaction that occurs simultaneously; one is an exergonic reaction that releases energy, and the other is an endergonic reaction that requires an input of energy in order to occur. 75.

endocytosis - 81

When cells take in substances by vesicle formation, the process is known as endocytosis.

Def: Process by which substances are moved into the cell from the environment by phagocytosis (cellular eating ) or pinocytosis (cellular drinking); includes receptor-mediated endosytosis. 81

energy - 72

Energy is defined as the capacity to do work -- to make things happen.

Def: Capacity to do work and bring about change; occurs in a variety of forms. 3, 72

energy law - 72

Two energy laws govern energy flow and help us understand the principles of energy consversion. The first law, called the law of conservation of energy, tell us: "Energy cannot be created or destroyed, but it can be changed frome one form to another."

The second energy law tells us : "Energy cannot be changed from one form to another without a loss of usable energy."

energy of activation (Ea) - 78

The energy needed to cause molecules to react with one another is called the energy of activation (Ea) Enzymes speed the rate of reactions because they lower the amount of energy required for the reactants to react.

Def: Energy that must be added in order for molecules to react with one another. 78

entropy - 73

The term entropy refers to the relative amount ogf disorganization. The only way to maintain or bring about order is to add more energy to a system.

Def: Measure of disorder or randomness. 73

enzyme - 77

Enzymes are protein molecules that function as organic catalyst to spread chemical reactions. Enzymes can only speed reactions that are possible to begin with. In the cell, an enzyme is analogous to a mutual friend who causes two people to meet and interact, because an enzyme brings together particular molecules and causes them to react with one another.

Def: Organic catalyst, usually a protein, that speeds a reaction in cells due to its particular shape. 40

enzyme inhibition - 78

Enzyme inhibition occurs when an active enzyme is prevented from combining with its substrate. Enzyme inhibitors are often poisonous to certain organisms. Cyanide, or example in an inhibitor of the enzyme cytochrome C oxidase which performs a vital function in cells because it is involved in making ATP.

Def: Means by which cells regulate enzyme activity; may be competitive or noncompetitive inhibition. 78

exocytosis - 81

Sometimes the molecules are too large to be moved by transport proteins. Instead, vesicle formation takes them into or out of a cell. Digestive enzymes and hormones are molecules transported out of the cell by exocytosis.

Def: Process in which an intracellular vesicle fuses with the plasma membrane so that the vesicle's contents are released outside the cell. 81

Def:

facilitated diffusion - 79

facilitated diffusion occurs when an ion or molecule diffuses across the membrane faster than expected, either by way of a specific channel protein or with the assistance of a carrier protein.

Def: Passive transfer of a substance into or out of a cell along a concentration gradient by a process that requires a carrier. 79

feedback inhibition - 78

The activity of almost every enzyme in a cell is regulated by feedback inhibition. When the end product is plentiful, it binds to a site other than the active site of the first enzyme in the pathway. This binding changes the shape of the active site, preventing the enzyme from binding to its substrate. Without the activity of the first enzyme, the entire pathway shuts down.

Def: Mechanism for regulating metabolic pathways in which the concentration of the product is kept within a certain range until binding shuts down the pathway, and no more product is produced. 78

heat - 73

Many forms of energy are usable, such as the energy of the sun, food, and ATP. Heat is diffused energy and the least usable form. Every energy conversion results in lost of usuable energy in the form of heat.

Def: Type of kinetic energy, captured solar genergy eventually dissipates as heat in the environment.

hypotonic solution - 80

Cells placed in a hypotonic solution (hypo, less than) gain water. Outside the cell, the concentration of solute is less, and the concentration of water is greater, than inside the cell. Animal cells placed in a hypotonic solution expand and sometimes burst.

DEF: Lower solute (more water) concentration than the cytoplasm of a cell, causes cell to gain water by osmosis. 80

hypertonic soution - 80

Cells placed in a hypertonic solution (hyper, more than) lose water. Outside the cell, the concentration of sulute is more, and the concentration of water is less, than inside the cell. Animal cells placed in a hypertonic solution shrink. For example, meats are sometimes, preserved by being salted. Bacteria are killed not by the salt, but by the lack of water in the meat.

Def: Higher solute concentration (less water) than the cytoplasm of a cell, causes cell to lose water by osmosis. 80

induced fit model -77

When the active site of an enzyme undergoes a slight change in shape in order to accommodate the substrate (s). This is called the induced fit model because the enzyme is induced to undergo a slight alteration to achieve optimal fit. In most instanced, only one small part of the enzyme called the active site accomodates the substrates, But sometimes the active site undergoes a sight change this is the induced fit model.

Def: Change in the shape of an enzyme's active site that enhances the fit between the active site and its substrates. 77

isotonic solution - 80

In the laboratory, cells are normally placed in isotonic solutions (iso, same as) in which the cell neither gains nor loses water. That is the concentration of water is the same on both sides of the membrane. In medical settings a 0.9% solution of the salt sodium chloride (NaCl) is known to be isotonic to red blood cells; therefore, intravenous solutions usually have this concentration.

Def: Solution that is equal to solute concentration to that of the cytoplasm of a cell causes cell to neither lose nor gain water by osmosis. 80

kilocalorie - 72

Kilocalories (1,000 calories)

Def: Caloric value of food: 1000 calories 72

kinetic energy -72

Potential energy is stored energy, and kinetic energy is the energy of motion. Potential energy is constantly being converted to kinetic energy, and vice versa.

Def: Energy associated with motion. 72

metobolic pathway. 77

Reactions do not occur haphazardly in cells. Just as a house is built in a series of ordered steps, chemical reactions in a cell usually occur in a particular order. They are part of the metabolic pathway, a series of linked reactions. Metabolic pathways begin with a particular reactant and terminate with an end product.

DEF: Series of linked reactions, beginning with a particular reactant and terminating with an end product. 77

osmosis - 79

Osmosis is the diffusion of water from the lower to the higher concentration of solute across the semipermeable membrane. A tube covered at the broad end by a differentially permeable membrane contains 5% salt solution, and the beaker contains only water. The salt ions are unable to pass through freely, but the water molecules can pass through the membrance. Therefore, there is a net movement of water toward the inside of the tube, where the percentage of water molecules is lower than the outside. the level of the solution rises in the tube because of the incoming water.

Def: Diffusion of water through a differentially permeable membrane.

phagocytosis - 81

When cells take in substances by vesicle formation, the process is known as endocytosis. If the material taken in is large, such as a food particle or another cell, the process is called phagocytosis. Phagocytosis is common in unicellular organisms, such as amoebas.

Pinocytosis occurs when vesicloes form around a liquid or around very small particles. The cell is getting a drink. White blood cells, cells that line the kidney tubules and intestinal wall, and plant root cells all use pinecytosis to ingest substances.

Def: Process by which vesicle formation brings macromolecules into the cell. 81

plasmolysis - 80

When a plant cell is placed in a hypertonic (hyper, more than) solution, the plasma membrane pulls away from the cell wall as the large central vacuole loses water. This is an example of plasmolysis, shrinking the cytoplasm due to osmosis. Cut flowers placed into salty water will wilt due to plasmolysis. The dead plants you may see along a roadside could have died due to exposure to a hypertonic solution during the winter, when salt was sued on the road.

Def: Contraction of the cell contents due to the loss of water. 80

potential energy - 72

The two basic forms of energy are potential energy and kinetic energy. Potential energy is stored energy, and kinetic energy is the energy of motion.

Def: Stored energy as a result of location or spatial arrangement. 72

receptor - mediated endocytosis - 81

During receptor-mediated endocytosis, receptors for particular substances are found at one location in the plasma membrane. This location is called a coated pit because there is a layer of protein on its intracellular side. Receptor-mediated endoytosis is selctive and much more efficient than ordinary pinocytosis. It is involved when substance move from maternal blood into fetal blood at the placenta . . .

Def: Selective uptake of molecules into a cell by vacuole formation after they bind to specific receptor proteins in the plasma membrane 81

simple diffusion - 79

No membrane is required for simple diffusion. During simple diffusion, molecules move down their concentration gradient until equilibrium is acheived and they are distrubted equally. Simple diffusion occurs because molecules are in motion, but it is a passive form of transport because a cell does not

need to expend energy for it to happen.

Def: Movement of molecules or ions from a region of higher to lower concentration; it requires no energy and tends to lead to an equal distribution. 79

sodium - potassium pump -81

Proteins that are engaged in active transport are often called pumps. The sodium-potassium pump, vitally important to nerve conduction, undergoes a change in shape that allows it to combine alternately with sodium ions and potassium ions.

Def: Carrier protein in the plasma membrane that moves sodium ions out of . . . and potassium ions into animal cells; important in nerve and muscle cells. 81

solute - 79

A solution contains both a solute and a solvent. In the case of the colored water. The dye is called the solute, and the water is called the solvent. Solutes are usually solids or gases, and solvents are usually liquids.

Def: Substance that is dissolved in a solvent, forming a solution. 79

substrate. - 77

The molecules acted on by an enzyme are called its substrates. An enzyme converts substrates into products. The substrates and products of an enzymatic reaction vary greatly. Many enzymes facilitate the breakdown of a substrate into multiple products. Or an enzyme may convert a single substrate into a single product. Still others may take two or more substrates and combine them into a single product.